In general, in the laundry dryer for automatic drying of wet washed laundry, there are exhaust type laundry dryers and condensing type laundry dryers.
Of the laundry dryers, the exhaust type dryers will be described.
FIG. 1 illustrates a diagram of a related art exhaust type dryer, and FIG. 2 illustrates a diagram of a flow passage of the dryer in FIG. 1.
The related art laundry dryer is provided with a body 1 having a door 2 in a front, a drum 3 rotatably mounted in the body 1 having a plurality of lifters 4 projected from an inside circumferential surface, driving means for providing rotating force to the drum 3, a heater 5 for heating external air introduced thereto to a high temperature, to produce a hot air, a suction duct 7 in communication with a rear opening of the drum 3 for guiding the hot air from the heater 5 to an inside of the drum 3, a lint duct 8 in communication with a front opening of the drum 3, for guiding humid air discharged after drying to an exhaust duct 15, and a fan 13 in rear of the lint duct 8 for generating blowing force.
Mounted to an inlet to the lint duct 8, there is a filter 14 for filtering foreign matter, such as lint, from air discharged from the drum 3.
The driving means for rotating the drum 3 is provided with a motor 10, and a driving belt 12 connected to a pulley 11 coupled to the motor 10 and wound around an outside circumferential surface of the drum 3, for rotating the drum 3 as the belt 12 wound on the driving pulley 11 rotates following rotation of the driving pulley by rotation of the motor 10.
Mounted to a front portion of the drum 3, there is an electrode sensor 30 for detecting a dryness of a drying object. The electrode sensor 30 has two metal plates arranged in parallel to each other, so that the electrode sensor 30 senses the dryness of the laundry with reference to an impedance generated at the opposite electrodes according to a water content of the drying object when the drying object is in contact with the opposite metal plates at the same time, and provides the dryness in a voltage signal.
That is, a microprocessor (so called micom) (not shown) which controls a general dryer system receives a the voltage signal from the electrode sensor 30, determines the dryness of the drying object with reference to a voltage level, and controls operation of the dryer according to this.
However, the direct contact type measurement of the dryness with the electrode sensor 30 fails to measure an accurate dryness due to a great deviation of the impedance coming from differences of impedances of various amounts of the drying objects, water contents, and kinds of the drying objects.
Moreover, an accurate sensor and a detecting circuit are required because, though the measurement of the dryness is easy owing to a great difference of the impedances varied at the time of initial drying when the drying object has much water content, the difference of the voltages provided is very small as the drying is progressed.
Moreover, the related art dryer, which finishes a course in a case a dryness sensed at the electrode sensor 30 reaches to a target dryness, fails to provide separate means for determining the amount of laundry.
That is, since the heater 5 is operated in a full capacity regardless of a load of the drying object, to provide hot air, energy more than necessary has been consumed in a drying course for a small load.
In a case an inverter control system is employed in which a speed of the motor 10 is varied freely, a sensing circuit including the electrode sensor 30, not only uses a power source the same with an inverter circuit, but also grounded to a ground terminal the same with the inverter circuit.
In this instance, since the inverter circuit is operated with utility AC power, and the sensing circuit is connected to the ground terminal the same with the inverter circuit without the power source being separated from each other, the sensing circuit has a high voltage applied thereto as it is.
That is, if a user opens the door and places a hand in the drum in a state the power source is not separated it is liable that an electric shock happens through the electrode sensor 30 and the laundry in contact with the electrode sensor 30.